Coke which is used in a blast furnace is required to be sufficiently strong, that is, to have sufficient strength in order to maintain gas permeability in the blast furnace. Since the coke is typically produced by carbonizing in a coke oven a coal blend which is prepared by blending plural brands of coal and the properties of such plural brands of coal strongly influence the strength of coke.
In a steel plant, a coal mine or the like, a coal is stored in a stock yard or etc. until the coal is used. During storage thereof, the coal is subjected to weathering as a result of being exposed to atmospheric air. It is known that the weathering influences properties of coal such as a caking property, amount of heat generation, and coking property of coal in various ways (see Non Patent Literature 1). In addition, it is reported that, in the case where coal (weathered coal) which has been subjected to weathering is added to a coal blend, there is an unexpected decrease in the strength of coke which is produced from such a coal blend (see Non Patent Literature 2). Thus, in the case where a weathered coal is added to a coal blend, there may be an unexpected decrease in strength. Therefore, evaluating the weathering degree of coal which influences the strength of coke has been attempted. Here, “weathering degree of coal” refers to the degree of change in the properties of coal due to weathering, and known examples of a method for evaluating the weathering degree of coal include (A), (B), and (C) below.
(A) Method for determining weathering degree by using a fluidity of coal as an index
“Fluidity of coal” refers to a property which is determined by using a method for determining the fluidity of coal by a Gieseler plastometer prescribed in JIS M 8801, in which maximum fluidity MF is used as an index. Since there is a tendency for maximum fluidity MF to decrease with an increase in the number of weathering days, and since maximum fluidity MF is used as one of the factors for controlling the quality of coke, in particular, the strength of coke, maximum fluidity MF has an advantage in that it is used as an index for both weathering degree and strength control (see Non Patent Literature 3).
(B) Method for determining weathering degree by using as an index the chemical composition of a gas which is generated when coal is heated
Patent Literature 1 proposes a method in which the weathering degree of coal is determined by using as indices the relationships among the contents of three constituents, that is, CH4, CO, and CO2 in a gas which is generated when coal is heated. With this method, it is possible to determine the weathering degree of coal whose MF is equal to or less than the detection limit due to weathering, and it is possible to estimate the amount of decrease in strength.
(C) Method for determining weathering degree by using the surface tension of coal as an index
Non Patent Literature 4 reports that it is possible to determine the weathering degree of coal by using as an index the surface tension of unheated coal which is determined by using a film flotation method. Non Patent Literature 4 describes that the surface tension of coal increases as weathering progresses.